chore: cleanup unused files and refine docs

[yorkie]

Checklist

• [ ] npm test passes
• [ ] tests and/or benchmarks are included
• [x] documentation is changed or added

[codecov[bot]]

Codecov Report

Merging #886 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #886   +/-   ##
=======================================
  Coverage   71.76%   71.76%           
=======================================
  Files          60       60           
  Lines        2430     2430           
=======================================
  Hits         1744     1744           
  Misses        686      686

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[legendecas]

Removed 'author ready' for changes requested.